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Tracing the evolutionary history of the Atlas flycatcher
(Ficedula speculigera) - A molecular genetic approach
Torbjørn Bruvik
Master of Science thesis Centre for Ecological and Evolutionary Synthesis
Department of Biology University of Oslo
2007
1
Forord
Blindern, 14.03.07 Da var tiden ute og oppgaven ferdig. Men det er selvsagt mange som bør takkes. Av alle som bør takkes er selvsagt min veileder Glenn-Peter Sætre først. Han har gitt meg en spennende oppgave, vært delaktig i alle deler av prosessen og gitt god veiledning hele veien. Thomas Borge, har hjulpet meg på laben i det skjulte, og har kommet med uvurderlig hjelp i analyse- og skriveprosessen. Stein Are Sæther har vært viktig i felt, og lært meg omtrent alt jeg kan i felt, men dessverre ikke alt han kan. Og sist, en stor takk til Gunilla Andersson, som har guidet meg gjennom laben, og gitt meg god faglig og ikke-faglig veiledning. Lesesalen har vært et fantastisk sted å være disse drøye to årene, med rom for faglige diskusjoner, fussball og andre ballspill, insektoppdrett og ellers mye tant og fjas (ikke så mye tant, da). Så jeg må takke de gamle storhetene Jostein, Petter, Irja, Kristina og Jens Ådne, og også mine likemenn Kjetil og Gry, og de senere ankomne jentene Silje, Elianne, Adine og Therese, som jeg har blitt som en far for. Også en takk til Guri som ofte har gitt meg den koffeinen, de beroligende ordene eller den ”sårende” mobbingen jeg trenger. Jeg må også takke mamma og pappa for noe økonomisk og mye moralsk støtte, og Hanne for litt korrekturlesing og mye råd fra sin rikholdige livserfaring.
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ABSTRACT................................................................................................................ 4
INTRODUCTION ........................................................................................................ 5
MATERIALS AND METHODS ................................................................................... 9
Fieldwork............................................................................................................................................................... 9
Phenotypic measures............................................................................................................................................. 9
DNA extraction...................................................................................................................................................... 9
PCR ...................................................................................................................................................................... 10
Sequencing ........................................................................................................................................................... 10
Data analysis........................................................................................................................................................ 10
RESULTS................................................................................................................. 12
Data ...................................................................................................................................................................... 12
Polymorphism and divergence........................................................................................................................... 13
Allele frequency spectra ..................................................................................................................................... 16
HKA-test .............................................................................................................................................................. 16
Shared and fixed polymorphisms ...................................................................................................................... 18
Phylogenetic reconstruction ............................................................................................................................... 20
Phenotypic measures........................................................................................................................................... 24
DISCUSSION ........................................................................................................... 25
The evolutionary history of Atlas flycatcher .................................................................................................... 25
The morphology of the Atlas flycatcher............................................................................................................ 26
Traces of introgression? ..................................................................................................................................... 26
Reduced variation at Z-linked loci .................................................................................................................... 27
Deviations from neutrality ................................................................................................................................. 30
Conservation issues ............................................................................................................................................. 30
REFERENCES ......................................................................................................... 32
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Abstract The Atlas flycatcher (Ficedula speculigera) is a poorly studied species in an otherwise
thoroughly studied species complex. In this study I attempt to reconstruct the evolutionary
history of this species, I look for possible traces of introgression, I test whether there is
contrasting patterns of polymorphism and divergence at autosomal and Z-linked genes as has
been found in other flycatcher species, and finally whether the Atlas flycatcher show reduced
genetic variation which would be expected due to recent habitat fragmentation. To address
these problems I used multilocus sequence analysis, with loci from both autosomes and Z-
chromosomes, and compared these with previously published sequences from two other
species, the pied flycatcher (F. hypoleuca) and the collared flycatcher (F. albicollis). Finally,
phenotypic measures of the Atlas flycatcher were compared with measures of these other two
species. The results appear consistent with a scenario in which an ancestral flycatcher species
became isolated in different refugia, presumably around the Mediterranean Sea, at the onset of
the Pleistocene glaciations and diverged into the present species. I found no traces of
introgression between the Atlas flycatcher and any of the two other species. Further, the Atlas
flycatcher showed reduced variation at Z-linked loci compared to autosomal loci, which may
indicate a complex demographic history or possibly selection. The Atlas flycatcher also
showed high variation compared to the other two species, and therefore seems to have a rather
large effective population size.
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Introduction Allopatric speciation, the evolution of reproductive isolation between populations that are
separated by a geographic barrier, is by many considered to be the “default” mode of
speciation (Coyne and Orr 2004). The simplest and most intuitive mode of allopatric
speciation is vicariant speciation, in which previously interbreeding populations get
geographically separated (Mayr 1942). This spatial separation can be caused by climatic or
geographic events. The isolated population would then diverge through genetic drift and
divergent natural selection. Consequently such isolated populations would accumulate genetic
differences over time that eventually would lead to reproductive isolation due to behavioural
and/or developmental incompatibilities and thus speciation. This process would tend to be
speeded up by divergent natural selection and countered or at least slowed down by gene
flow.
A recent source of vicariance for many organisms, was the climate cycles of the Pleistocene
(1,808,000 to 11,550 years before present) (Hewitt 1996; Avise and Walker 1998; Avise et al.
1998). For instance, with recurrent glaciation in Europe many forest-dwelling species were
pushed back to isolated refuge areas located to the peninsulas bordering the Mediterranean
Sea (i.e. Iberia, Italy and the Balkans) as well as North Africa, where the forest habitat
remained during the cold periods (Hewitt 1996; Avise and Walker 1998; Avise et al. 1998).
After the retraction of the ice, the continent was re-colonised, sometimes resulting in
populations from different refugia experiencing secondary contact. Consistent with this
scenario, we find higher genetic diversity in many organisms in the area around the
Mediterranean Sea than in other parts of their breeding ranges, and also hybrid zones further
north reflecting the edge of range of dispersal after the retraction of the ice (Hewitt 1996).
The black-and-white Ficedula flycatcher species complex appears to have been strongly
affected by vicariance episodes during the Pleistocene glaciations (Sætre et al. 2001a). The
taxonomic relations and evolutionary history of the species have been much debated
(Lundberg and Alatalo 1992). The species complex was previously assumed to consist only of
two species, the pied flycatcher (Ficedula hypoleuca) and the collared flycatcher (F.
albicollis). However, sequence comparisons of mitochondrial genes from different flycatcher
populations have shown that there are four highly divergent groups (Sætre et al. 2001a; Sætre
et al. 2001b). Accordingly, it has been argued that the semicollared flycatcher (F.
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semitorquata), earlier assumed to be a subspecies of the collared flycatcher, and the Atlas
flycatcher (F. speculigera), earlier assumed to be a subspecies of the pied flycatcher, should
also be regarded as distinct species (Sætre et al. 2001a; Sætre et al. 2001b). These species
have been argued to originate from an ancestral population that became isolated in different
refuge areas around the Mediterranean Sea approximately 2 million years ago, at the onset of
the Pleistocene glaciations. Following the retraction of the ice, the pied and the collared
flycatcher expanded northwards from their respective refugia on the Iberian and Italian
peninsulas and eventually came into secondary contact in Central and Eastern Europe,
whereas the semicollared and the Atlas flycatcher remained close to their assumed refuge
areas respectively in Balkan/Caucasus and the Atlas mountains of North Africa (Sætre et al.
2001a).
Figure 1 – The present geographical breeding distributions of the four species of Ficedula flycatchers.
6
The Ficedula flycatchers are forest dwelling passerine birds in the Muscicapidae family. They
are all sexually dimorphic and the four species share many features phenotypically and
behaviourally. They are long distance migrants with breeding grounds in Europe and north-
western Africa (figure 1) and wintering grounds in tropical Africa (Lundberg and Alatalo
1992). In the areas where pied and collared flycatchers live in sympatry (central and eastern
Europe and the Baltic Isles) hybridisation occurs. They breed in nest holes in trees, which
limit them to some extent to older forest. Ficedula flycatchers are easy to study, since they
prefer nest boxes to natural nest holes, and are fairly tame and easily handled for blood
samples and phenotypic measures. Consequently the pied and collared flycatchers have been
popular study objects for behavioural, ecological and in later years also evolutionary genetic
studies. However the two other species are poorly studied.
This study focuses on the Atlas flycatcher specifically. The Atlas flycatcher has its breeding
grounds in the Atlas Mountains of Morocco, Algeria and Tunisia, where it is found primarily
in old deciduous forest. The study consists of a multilocus sequence analysis of Atlas
flycatchers, which are compared with previously published sequences of pied and collared
flycatchers, and of the red-breasted flycatcher (Ficedula parva) which serve as an outgroup
(Borge et al. 2005). I also present descriptive statistics of various phenotypic measures of the
species to compare with data from pied and collared flycatchers.
The few previous studies of the Atlas flycatcher are based on few individuals, and mainly
mitochondrial DNA has been analysed. The aim of this study is to do a more thorough genetic
investigation of the species, using a larger sample size and to focus on nuclear genes. The
nuclear and mitochondrial genomes may provide different signals from a species’
evolutionary history (Shaw 2002; Bensch et al. 2006). For instance following Haldane’s rule,
since in birds females are the heterogametic sex, gene flow between incipient species may be
brought to a halt at an earlier stage at the mitochondrial than at the nuclear genome. This is
because mitochondria are inherited through the maternal line and since females are expected
to be the sex first experiencing hybrid incompatibilities in species with female heterogamy
(Coyne and Orr 2004). Accordingly, historical contact (gene flow) between the Atlas
flycatcher and other flycatcher species may be more likely to be detected using nuclear
genetic markers than mtDNA. So far, the taxonomic status of the Atlas flycatcher is only
based on a rather limited comparison of mtDNA. It is therefore of interest to investigate other
genetic markers to check whether or not one finds concordant results.
7
In a previous study of pied and collared flycatchers Borge et al. (2005) found contrasting
patterns of polymorphism and divergence at autosomal and Z-linked genes. They suggested
that selective sweeps on the Z-chromosome could explain this pattern and that such selection
events could have been important during their speciation. Here, I compare sequences both
from autosomal and Z-linked genes to investigate whether or not I find similar patterns in the
Atlas flycatcher. Moreover, since the effective population size of a Z-linked gene is ¾ of that
of an autosomal gene, demographic processes, such as changes in population size, may affect
Z-linked and autosomal genes differently and thus provide additional information for
reconstructions of the demographic history of the species. At present, the Atlas flycatcher
appears to have a very patchy and fragmented distribution due to strong deforestation in its
breeding range. Hence, one may expect reduced genetic variation if its population structure
has been negatively affected by this habitat fragmentation. As such, this study may also have
applications for conservation issues of the species.
8
Materials and methods
Fieldwork The fieldwork was conducted from 3rd to 13th of May 2005 near the towns of Asrou and Ifrane
(33°N, 5°W) located in the Middle Atlas Mountains of Morocco, at around 1500AMSL
(above main sea level). The breeding localities here are scattered in patches of old deciduous
forest, mainly oak (Quercus spp), with ample supply of breeding holes. 33 birds (30 males
and 3 females) were caught using mist nets and male song playback, and all birds were
measured for weight, size and plumage characteristics. In addition, data on 5 males caught in
the same area during May 1998 were available. 25 μl of blood were collected in capillary
tubes after puncturing the brachial vein and suspended in Queen’s Lysis Buffer (Seutin et al.
1991).
Phenotypic measures All birds caught were measured for plumage characteristics. The characteristics measured
include the colour score according to the seven-point scale of Drost (1936) where score 1 =
completely black head and score 7 = completely brown, height of the white forehead patch,
the number of white-edged tail feathers, the maximum wing patch width, further I noticed at
which primary the wing patch started, the rump colour and whether a neck collar was found or
not. All length, height and width estimates were measured with a dial calliper with an
accuracy of 0.1 mm.
DNA extraction 40 μl blood and buffer mix were mixed with 500 μl of 0.1 M NaCl, 10 mM Tris-Cl (pH 8.0),
5% SDS and 50 μl proteinase K (10 mg/ml) and incubated overnight on a thermoshaker. The
samples were treated twice with phenol/chloroform/isoamylalcohol (Sambrook et al. 1989)
and the DNA was precipitated with 96% ethanol and cleansed with 70% ethanol twice, before
the pellets were dried and redissolved in TE buffer (10 mM Tris, 1 mM EDTA, pH 7.5). 9
male individuals were selected for further analysis. Since comparing autosomal with Z-linked
loci is an essential part of this study, only males were used. This is because males are the
homogametic sex, and therefore possess two Z-chromosomes, and the comparison will be
more balanced.
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PCR Several primer pairs that has been extensively tested on Ficedula flycatchers were available
(Borge et al. 2005). These were designed from GenBank entries on chicken (Gallus gallus). In
short, primers were designed in exon sequence flanking introns of appropriate size from both
autosomal and Z chromosome-linked genes. In cases of variable amplification success or for
long introns, new flycatcher-specific primers were designed (Borge et al. 2005). For the
present study I tested most primer pairs that had high amplification success in the earlier study
on the pied and collared flycatcher (Borge et al. 2005), and chose the ones in which readable
sequences were obtained in all individuals for further analysis. These loci are listed in Table
1.
The amplification reaction had a total volume of 10 μl and contained 1X PCR Buffer II
(Applied Biosystems), 0.2 mM of each dNTP, 0.32 μM of each primer, 1 μg of bovine serum
albumin (BSA), 0.3 units of AmpliTaq DNA polymerase (Applied Biosystems), 1 mM MgCl2
and 10 ng DNA. The amplifications were run on a PTC-200 or a DNA Engine Tetrad 2 (MJ
Research, Waterton, MA, USA) with the following profile: 95°C for 1 minute and then 35-40
cycles of 94°C for 30 seconds, 50-62°C for 30 seconds, 72°C for 1 minute. Finally an
extension step of 10 minutes at 72°C was used.
Sequencing Excess nucleotides and primers were removed from the PCR-products using ExoSap-IT
(United States Biochemical, Cleveland) and then cycle sequenced using BigDye Terminator
sequencing buffer and v3.1 Cycle Sequencing kit (Applied Biosystems) and analyzed on an
ABI 3730 DNA Analyzer (Applied Biosystems). Sequences were aligned and edited with
MEGA 3.1 (Kumar et al. 2001) and manually adjusted. To get reliable sequences, bases were
called on the basis of both strands in all instances, except short regions where one direction
was of low quality, due to short insertion/deletions (indels) or sequence repeats. In such
instances the sequence was called based on primarily multiple sequences of one strand.
Data analysis The diploid sequences from the 9 Atlas flycatchers were transformed into “pseudo-
haplotypes” together with sequences from 9 collared flycatchers, 9 pied flycatchers and one
redbreasted flycatcher. This was done by assigning the two alleles at each polymorphic site to
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one of the two sequences, on the basis of the ambiguity codes produced. The sequences were
aligned using ClustalW (Thompson et al. 1994) and manually adjusted.
Analysis of polymorphism and divergence were done using the program DnaSP 4.0 (Rozas
and Rozas 1999). This program gives basic descriptive statistics of the sequences, estimates of
the population mutation rate (nucleotide diversity), π (Nei 1987), and θ (Watterson 1975), and
their standard deviation. It also performs tests of neutrality based on the allele frequency
spectrum, such as Tajima’s D (Tajima 1989b) and Fu and Li’s D and F (Fu and Li 1993).
Multilocus Hudson-Kreitman-Aguadé (HKA) tests (Hudson et al. 1987) were done using the
HKA program (http://lifesci.rutgers.edu/~heylab/). This test investigates if interspecific and
intraspecific variation is positively correlated as expected under neutral evolution. To assess
whether the different population pairs had originated from an ancestral larger population
without later gene flow, isolation model fitting was done as described in Wakeley and Hey
(1997) and Wang et al. (1997) using the WH program (http://lifesci.rutgers.edu/~heylab/).
Neighbour-joining trees were constructed using the program MEGA 3.1 (Kumar et al. 2001),
to see if a larger sample size of nuclear gene gives the same topography as mtDNA and fewer
nuclear genes has done earlier (Sætre et al. 2001a; Sætre et al. 2001b).
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Results
Data The sequences generated from the 9 Atlas flycatchers were analysed together with sequences
generated in a previous study (Borge et al. 2005), namely 9 collared flycatchers from Italy, 9
pied flycatchers from Spain and one red-breasted flycatcher. Since pseudohaplotypes were
constructed, and since only males were used, the sample size per locus is 18 for both Z-linked
genes and autosomal genes. In the present study 16 loci were used, 10 of which were Z-linked
genes (4189 bp in total) and 6 of which were autosomal genes (3335 bp in total). Some of the
sequences possessed short insertion-deletions (indels) across the three species. These short
regions were excluded from the analysis to get the same sequence length across the species
and individuals for easier comparisons. Because all sequences of one locus had to be the same
length, the sequences were all cut down to the same length as the shortest sequence in the
sample, resulting in a loss of sequence data. The sequences are mostly intronic, though some
coding sequences at the ends are featured in the analysis. Some further details of these loci are
presented in table 1.
Table 1 - Sample sequences
Length
Cytological
position Total Exon Intron ALDOB-6 Z 460 23 437 ALDOB-7 Z 153 8 145 BRM-12 Z 1572 0 1572 CHDZ-15 Z 495 34 461 CHDZ-18 Z 209 0 209 GHR-5.1 Z 112 0 112 GHR-5.2 Z 450 0 450 VLDLR-8 Z 127 59 68 VLDLR-9 Z 409 33 376 VLDLR-12 Z 196 72 124 ACLY-16 A 358 3 355 ALAS1-8 A 291 0 291 FAS-Y A 901 0 901 RHO-1 A 375 0 375 RPL30-3 A 1008 0 1008 TGFB2-5 A 402 0 402
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Polymorphism and divergence Data on polymorphism are summarised in table 2 and figure 2. Loci from the same gene are
combined and analysed as one gene. Several measures of polymorphism were used, including
π (Nei and Li 1979), the number of nucleotide differences per site between pairs of randomly
chosen sequences, and Watterson’s estimate of the population mutation rate parameter θW
(Watterson 1975). The two measures of polymorphism showed similar patterns, with the pied
flycatcher exhibiting the lowest level of variation, and rather similar and higher variation in
the collared and Atlas flycatchers (Table 2). Divergence from the outgroup was very similar
for all species and at all loci, indicating a similar rate of mutation in all three species. The
above-mentioned differences in variation are therefore likely to reflect differences in effective
population size, Ne.
The divergence from the outgroup was somewhat higher at autosomal loci compared to Z-
linked loci in both the Atlas flycatcher (0.0189 at Z-linked loci and 0.0195 at autosomal loci)
and the pied flycatcher (0.0189 at Z-linked loci and 0.0195 at autosomal loci), but not in the
collared flycatcher (0.0193 at Z-linked loci and 0.0192 at autosomal loci).
Levels of polymorphism were found to be distinctly lower on the Z-linked loci compared to
the autosomal loci (Table 2). The levels of variation at the Z-linked were 60%, 48% and 51%
that of the autosomal variation in Atlas flycatcher, collared flycatcher and pied flycatcher,
respectively. One would expect higher mutation rate at the Z chromosome to lead to higher
variation at this chromosome than at autosomal ones. This would on the other hand be
counteracted by ¾ because of the lower effective population size of the Z-chromosome since
females only have one copy of the chromosome. Note, however, that these expectations only
apply under the assumptions of a standard neutral model, i.e. assuming constant population
size, random mating, no selection and no migration (Watterson 1975).
The expected variation at Z-linked loci based on the divergence in autosomal loci was
calculated as suggested by Borge et al. (2005), using the following equation:
A
AZZ D
SDS
43
(exp) =
where SZ(exp) is the expected number of segregating sites at a Z-linked locus, DZ is the
divergence between the focal species (i.e. Atlas, collared or pied flycatcher) and the outgroup
13
(red-breasted flycatcher), SA is the total number of segregating sites at autosomal loci, and DA
is the total divergence between the three species and the outgroup at autosomal loci. A
corresponding formula was used to calculate the expected number of segregating sites at a
specific autosomal locus, SA(exp):
Z
ZAA D
SDS3
4(exp) =
These equations were used to calculate the expected S-values, and the results are presented in
table 2. At Z-linked loci the observed S-values are lower than the ones predicted from
polymorphism and divergence at autosomal loci. This was seen in 12 of 15 cases (signs test,
P=0.0176). At autosomal loci on the other hand, the observed number of segregating sites
were higher than the predicted in 11 of 18 cases (signs test, P=0.2403). These data therefore
suggest that variation at Z-linked loci is reduced compared to autosomal loci under a standard
neutral model, although the analysis of the autosomal loci was not significant (see also figure
2).
0
0,002
0,004
0,006
0,008
0,01
0,012
0,014
Z A Z A Z A Z A Z A Z A Z A Z A Z A
Pol MA Pol IC Pol SP Shared MA-IC Shared MA-SP Shared IC-SP Fixed MA-IC Fixed MA-SP Fixed IC-SP
Figure 2 – Descriptive statistics of polymorphism and divergence for Atlas flycatcher (MA), collared flycatcher (IC) and pied flycatcher (SP) at Z-linked loci (4182 bp) and autosomal loci (3334 bp). Z-linked loci are shown in green and autosomal loci in blue. The number of within species polymorphisms (abbreviated Pol), between species shared polymorphisms (Shared) and fixed differences between species (Fixed) are all shown as per base pair.
14
Table 2 – Polymorphism summary
* P<0.05
Gene Speciesa Lb Kc Sd Sexpe sf πg SD θw
h SD DTi DFL
j FFLk
Aldob MA 620 14 0 5.8154 0 0.0000 0.0000 0.0000 0.0000 0.00 0.00 0.00 IC 620 14 1 7.0547 0 0.0003 0.0002 0.0005 0.0005 -0.53 0.67 0.40 SP 620 13 3 4.2000 1 0.0013 0.0003 0.0014 0.0009 -0.26 -0.08 -0.15Brm MA 1572 37 15 15.3692 8 0.0017 0.0004 0.0028 0.0012 -1.52 -1.11 -1.42 IC 1572 41 9 20.6602 5 0.0013 0.0002 0.0017 0.0008 -0.72 -1.11 -1.16 SP 1571 38 10 12.2769 0 0.0028 0.0002 0.0019 0.0008 1.79 1.41* 1.75*Chdz MA 704 14 1 5.8154 0 0.0006 0.0001 0.0004 0.0004 0.87 0.67 0.82 IC 704 13 1 6.5508 1 0.0002 0.0001 0.0004 0.0004 -1.16 -1.50 -1.61 SP 704 14 2 4.5231 0 0.0007 0.0002 0.0008 0.0006 -0.33 0.88 0.64Ghr MA 555 9 3 3.7385 0 0.0022 0.0003 0.0016 0.0010 1.18 1.02 1.22 IC 562 9 2 4.5352 1 0.0011 0.0002 0.0010 0.0008 0.25 -0.55 -0.39 SP 562 10 0 3.2308 0 0.0000 0.0000 0.0000 0.0000 0.00 0.00 0.00Vldlr MA 731 5 8 2.0769 4 0.0022 0.0005 0.0032 0.0015 -1.07 -0.84 -1.05 IC 731 4 13 2.0156 3 0.0058 0.0006 0.0052 0.0022 0.47 0.37 0.46
SP 731 4 3 1.2923 0 0.0009 0.0005 0.0012 0.0008 -0.78 1.02 0.62
Total MA 4182 79 27 32.8154 12 0.0014 0.0002 0.0019 0.0007 -1.00 -0.72 -0.93 IC 4189 81 26 40.8164 10 0.0017 0.0002 0.0018 0.0007 -0.14 -0.40 -0.38
SP 4188 79 18 25.5231 1 0.0015 0.0001 0.0013 0.0005 0.77 1.27 1.31
Acly-16 MA 358 5 0 2.2785 0 0.0000 0.0000 0.0000 0.0000 0.00 0.00 0.00 IC 358 5 1 2.1399 1 0.0003 0.0003 0.0008 0.0008 -1.16 -1.50 -1.61 SP 358 5 2 1.5190 2 0.0006 0.0004 0.0016 0.0012 -1.51 -1.99 -2.13Alas1-8 MA 291 12 5 5.4684 1 0.0047 0.0006 0.0050 0.0027 -0.22 0.42 0.28 IC 291 14 7 5.9918 3 0.0055 0.0009 0.0070 0.0035 -0.75 -0.51 -0.67 SP 291 15 4 4.5570 2 0.0022 0.0008 0.0040 0.0023 -1.35 -0.70 -1.01Fas-y MA 900 17 2 7.7468 1 0.0004 0.0002 0.0007 0.0005 -1.10 -0.55 -0.80 IC 901 17 2 7.2757 0 0.0006 0.0002 0.0007 0.0005 -0.33 0.88 0.64 SP 900 16 0 4.8608 0 0.0000 0.0000 0.0000 0.0000 0.00 0.00 0.00Rho-1 MA 375 9 4 4.1013 1 0.0032 0.0006 0.0031 0.0018 0.09 0.21 0.20 IC 375 7 7 2.9959 1 0.0062 0.0008 0.0054 0.0027 0.51 0.70 0.75 SP 375 7 7 2.1266 1 0.0067 0.0009 0.0054 0.0027 0.81 0.70 0.84Rpl30-3 MA 1008 17 19 7.7468 9 0.0055 0.0004 0.0055 0.0022 -0.01 -0.84 -0.70 IC 1008 16 20 6.8477 4 0.0053 0.0008 0.0058 0.0023 -0.29 0.55 0.36 SP 1008 17 12 5.1646 6 0.0027 0.0008 0.0035 0.0015 -0.86 -0.91 -1.04Tgfb2-5 MA 402 5 6 2.2785 2 0.0040 0.0006 0.0043 0.0022 -0.25 -0.10 -0.17 IC 402 5 6 2.1399 4 0.0028 0.0007 0.0043 0.0022 -1.21 -1.47 -1.61
SP 402 5 3 1.5190 1 0.0023 0.0004 0.0022 0.0014 0.12 -0.08 -0.04
Total MA 3334 65 36 29.6203 14 0.0030 0.0002 0.0031 0.0012 -0.19 -0.44 -0.42 IC 3335 64 43 27.3909 13 0.0033 0.0004 0.0038 0.0014 -0.48 0.03 -0.14 SP 3334 65 28 19.7468 12 0.0021 0.0003 0.0024 0.0010 -0.57 -0.64 -0.72
a MA, Moroccan Atlas flycatcher; IC, Italian collared flycatcher; SP, Spanish pied flycatcher. b Number of sites surveyed. c Divergence with outgroup d Number of segregating sites e The expected number of segregating sites based on Ne estimated from autosomal or Z-linked loci respectively. f Number of singleton sites. g Average pairwise sequence difference per nucleotide (Nei 1987). h Expected heterozygosity per nucleotide (Watterson 1975). i (Tajima 1989b). j (Fu and Li 1993). k (Fu and Li 1993).
15
Allele frequency spectra Three different tests of neutrality were used (Table 2); Tajima’s D (1989b) and Fu and Li’s D
and F (1993). In most cases the tests yielded slightly negative test statistics albeit not
significantly different from zero. Negative test statistics means that there is an excess of rare
variants compared to neutral expectation. The pied flycatcher, however, showed positive test
statistics at several Z-linked loci, and significantly so at the locus BRM at Fu and Li’s D and
F, and a near significant at Tajima’s D. The overall test statistics for the Z-linked loci were
also positive, and borderline significant at Fu and Li’s D value. This indicates a deficit of rare
variants compared to neutral expectation.
HKA-test Multilocus HKA-tests were run for all species pairs for both Z-linked and autosomal loci
(Table 3). In all tests the polymorphism and divergence data from table 2 and 4 were used.
The HKA-test tests whether variation within and between species are positively correlated
(Hudson et al. 1987), as expected under neutrality. 10,000 coalescent simulations were run to
assess significance. None of the species pair combinations yielded significant deviations from
the neutral expectation at neither autosomal nor Z-linked loci. In a previous study of pied and
collared flycatcher using a large number of loci, significant deviations from neutrality was
found at Z-linked loci (Borge et al. 2005). Consistent with this result I did find slightly higher
deviations from neutrality at the Z-linked loci than at the autosomal ones, although neither
comparison was significant.
16
Table 3 - HKA-results Segregating sites Divergence Atlas flycatcher Collared flycatcher Gene Obsa Expb Obs Exp Obs ExpALDOB 0 1.97 1 1.9 4.89 2.02BRM 15 12.13 9 11,68 12.2 12.4CHDZ 1 1.12 1 1.08 1.33 1.14GHR 3 2.29 2 2.2 1.83 2.34VLDLR 8 9.49 13 9.14 7.33 9.7ACLY 0 0.37 1 0.44 0.06 0.25ALAS1 5 5.33 7 6.37 3.33 3.64FAS 2 2.93 2 3.51 4.44 2RHO 4 4.86 7 5.81 2.98 3.31RPL30 19 17.79 20 21.25 12.18 12.13TGFB2 6 4.71 6 5.63 1.56 3.21
Segregating sites Divergence Atlas flycatcher Pied flycatcher Gene Obs Exp Obs Exp Obs ExpALDOB 0 1.96 3 1.31 1.83 1.56BRM 15 14.54 10 9.68 10.82 11.6CHDZ 1 1.44 2 0.96 0.56 1.15GHR 3 2.34 0 1.58 2.78 1.86VLDLR 8 6.72 3 4.48 5.56 5.36
ACLY 0 0.86 2 0.67 0.11 0.58ALAS1 5 4.97 4 3.87 3.17 3.32FAS 2 1.29 0 1.01 1.17 0.86RHO 4 7.11 7 5.53 6.39 4.75RPL30 19 17.35 12 13.49 11.43 11.59TGFB2 6 4.41 3 3.43 1.78 2.94
Segregating sites Divergence Collared flycatcher Pied flycatcher Gene Obs Exp Obs Exp Obs ExpALDOB 1 2.31 3 1.6 1.83 1.92BRM 9 11.83 10 8.19 10.82 9.8CHDZ 1 1.41 2 0.98 0.56 1.17GHR 2 1.9 0 1.31 2.78 1.57VLDLR 13 8.55 3 5.92 5.56 7.08
ACLY 1 1.91 2 1.25 1.17 1.01ALAS1 7 7.34 4 4.78 4.97 3.85FAS 2 2.43 0 1.58 3.28 1.27RHO 7 8.6 7 5.6 4.72 4.52RPL30 20 17.95 12 11.69 7.07 9.43TGFB2 6 4.77 3 3.1 1.38 2.5
a Observed; b Expected.
17
Shared and fixed polymorphisms The polymorphisms were sorted into four different categories in each of the three species pair
combinations; polymorphisms exclusive to species 1, polymorphisms exclusive to species 2,
fixed substitutions between species 1 and 2, and shared polymorphisms between species 1 and
2 (Table 4). I found an interesting difference between the shared polymorphisms and the fixed
substitutions at the Z-linked and the autosomal loci. Z-linked genes exhibit almost no shared
polymorphisms but a high number of fixed substitutions. Autosomal genes show the opposite
pattern, although less clear cut, with a higher number of fixed substitutions than shared
polymorphisms.
Isolation model fitting was performed on the data, with all autosomal and all Z-linked genes
grouped together as two “loci”. The isolation model is a simple speciation model (Wakeley
and Hey 1997) that assumes that two populations arose from one ancestral population some
time, t generations in the past. The model further assumes neutral mutations, no gene flow and
constant population sizes, except at the separation time. Four parameters describe the isolation
model, three values for the population mutation parameter θ: θA for the ancestral population,
θ1 for population 1 and θ2 for population 2. In addition there is the time of the speciation event
T, measured in units of 2N1 generations (Wakeley and Hey 1997; Wang et al. 1997). With
these assumptions random drift would cause shared polymorphisms to be lost and become
fixed differences with time. The tests were performed between all three species pair, with
10,000 coalescent simulations to assess significance, and the results can be found in table 5.
None of the tests showed significant deviations from the isolation model (WH-values of 12, 8
and 16 and P-values of 0.65, 0.79 and 0.61 respectively), and thus the model with no selection
and gene flow is not rejected.
18
Table 4 - Fixed and shard polymorphisms
Fixed and shared polymorphism between MA and IC Sx-MA Sx-IC Sfixed Sshared Saverage length ALDOB 0 1 4 0 4.889 620 BRM 14 8 8 1 12.204 1572 CHDZ 1 1 1 0 1.333 704 GHR 2 1 0 1 1.833 555 VLDLR 7 12 0 1 7.333 731 Total-Z 24 23 13 3 27.593 4182 ACLY 0 1 0 0 0.056 358 ALAS1 5 7 0 0 3.333 291 FAS 2 2 4 0 4.444 900 RHO 2 5 0 2 2.981 375 RPL30 15 16 0 4 12.179 1008 TGFB2 6 6 0 0 1.556 402 Total-A 30 37 4 6 24.549 3334
Fixed and shared polymorphism between MA and SP Sx-MA Sx-SP Sfixed Sshared Saverage length ALDOB 0 3 1 0 1.833 620 BRM 13 8 5 2 10.815 1571 CHDZ 1 2 0 0 0.556 704 GHR 3 0 1 0 2.778 555 VLDLR 8 3 1 0 5.556 731 Total-Z 25 16 8 2 21.538 4181 ACLY 0 2 0 0 0.111 358 ALAS1 5 4 1 0 3.167 291 FAS 2 0 1 0 1.167 900 RHO 4 7 3 0 6.389 375 RPL30 11 4 1 8 11.432 1008 TGFB2 6 3 0 0 1.778 402 Total-A 28 20 6 8 24.044 3334
Fixed and shared polymorphism between IC and SP Sx-IC Sx-SP Sfixed Sshared Saverage length ALDOB 1 3 3 0 4.722 620 BRM 8 9 5 1 11.247 1603 CHDZ 1 2 1 0 1.333 804 GHR 2 0 2 0 2.556 598 VLDLR 13 3 1 0 4.611 732 Total-Z 25 17 12 1 24.469 4357 ACLY 1 2 1 0 1.167 417 ALAS1 13 5 0 2 4.969 403 FAS 2 0 3 0 3.278 900 RHO 6 6 1 1 4.716 404 RPL30 15 7 0 6 7.074 1065 TGFB2 5 2 0 1 1.377 402 Total-A 42 22 5 10 22.581 3591
S-values are the number of variable sites, where the subscripts indicate those exclusive (x) to the Atlas flycatcher (Sx-MA), the collared flycatcher (Sx-IC) or the pied flycatcher (Sx-SP), fixed differences (Sfixed) or shared polymorphisms between the species (Sshared), and the average pairwise differences (Saverage) between the species.
19
Table 5 - Isolation model fitting
θ1 θ2 θA T WH P MA/IC 13.225 14.564 50.244 0.573 12.000 0.660 MA/SP 9.752 7.056 48.822 0.432 8.000 0.793 IC/SP 12.253 7.795 56.870 0.417 16.000 0.615 θ1 and θ2 are the estimates of θ for population 1 and 2 of the three species pairs, and θA is the estimate for their common ancestral population. T is the estimate of time since the speciation event. WH is a test statistic designed for this model (Wang et al. 1997) and P is the probability calculated from this test statistic.
Phylogenetic reconstruction On the basis of all the Z-linked genes, all the autosomal genes and finally all loci combined,
three neighbour-joining trees were constructed (figure 3-5). The trees were constructed from
the above mentioned pseudo-haplotype data. The three different consensus trees showed a
slightly different topography for the branching events of the three species. For the Z-linked
genes the collared flycatcher branch off before the other two, whereas for the autosomal genes
as well as all loci combined the Atlas flycatcher is the first to branch off. However, in all
cases, the bootstrap support for the branching events of the three species is low, and the
branch lengths short, suggesting that the three species diverged from each other
approximately simultaneously. A similar result was found in previous studies using mtDNA
sequence data (Sætre et al. 2001a; Sætre et al. 2001b).
20
Figure 3 – Neighbour-joining consensus tree with bootstrap values of all Z-linked loci
SP9 F
SP9 R
SP6 F
SP6 R
SP21 R
SP19 R
SP4 R
SP19 F
SP8 F
SP4 F
SP8 R
SP24 R
SP11 R
SP24 F
SP11 F
SP23 F
SP23 R
SP21 F
MA3 F
MA3 R
MA1 F
MA6 F
MA9 F
MA2 F
MA2 R
MA6 R
MA4 R
MA8 R
MA1 R
MA9 R
MA7 F
MA7 R
MA4 F
MA5 R
MA5 F
MA8 F
IC10 R
IC7 F
IC5 F
IC9 F
IC10 F
IC5 R
IC4 F
IC4 R
IC7 R
IC3 F
IC2 F
IC6 F
IC3 R
IC2 R
IC9 R
IC6 R
IC8 F
IC8 R
Parv F
Parv R
99
98
70
46
43
29
28
40
3536
25
24
13
30
17
31
99
99
92
88
87
56
52
32
32
50
48
44
19
11
16
6
3
25
98
46
56
48
4744
99
3322
18
43
16
11
4
317
2
2
23
0.002
21
Figure 4 – Neighbour-joining consensus tree with bootstrap values of all autosomal loci
SP19 R
SP9 R
SP23 R
SP6 R
SP24 R
SP8 R
SP9 F
SP4 F
SP4 R
SP21 F
SP21 R
SP6 F
SP24 F
SP23 F
SP8 F
SP19 F
SP11 F
SP11 R
IC10 F
IC8 F
IC9 R
IC8 R
IC9 F
IC2 F
IC10 R
IC6 F
IC6 R
IC2 R
IC7 F
IC5 F
IC4 F
IC4 R
IC7 R
IC5 R
IC3 F
IC3 R
MA8 F
MA7 F
MA2 F
MA2 R
MA3 F
MA3 R
MA4 F
MA7 R
MA4 R
MA8 R
MA1 R
MA1 F
MA5 F
MA9 F
MA9 R
MA5 R
MA6 F
MA6 R
Parv F
PArv R99
47
42
40
36
32
10
7
5
5
1
8
4
9
66
61
53
33
31
26
22
12
13
19
10
13
46
35
62
47
65
50
47
36
4096
11
23
28
38
47
53
30
28
46
0.002
22
Figure 5 – Neighbour-joining consensus tree with bootstrap values of all loci combined
SP9 F
SP9 R
SP21 R
SP19 R
SP6 F
SP6 R
SP4 R
SP24 R
SP8 R
SP19 F
SP8 F
SP4 F
SP24 F
SP21 F
SP23 F
SP23 R
SP11 F
SP11 R
IC10 F
IC10 R
IC9 F
IC8 F
IC9 R
IC8 R
IC6 F
IC2 F
IC6 R
IC2 R
IC7 R
IC3 R
IC5 R
IC7 F
IC5 F
IC3 F
IC4 F
IC4 R
MA3 F
MA7 F
MA2 F
MA2 R
MA3 R
MA4 F
MA8 F
MA7 R
MA4 R
MA8 R
MA1 R
MA5 R
MA6 R
MA9 R
MA1 F
MA6 F
MA5 F
MA9 F
Parv F
Parv R
70
59
50
49
45
45
35
32
30
21
10
8
9
4
18
99
99
60
59
46
42
22
21
39
29
28
9
3
34
97
62
75
54
50
31
9919
292
9
10
2143
36
43
10
36
24
0.002
23
Phenotypic measures The phenotypic measures were compared with similar measures from collared and pied
flycatchers published by Sætre et al. (2001b). For most of the plumage characteristics the
Atlas flycatcher is more similar to the collared flycatcher (Table 6) than to the pied flycatcher.
Both the Atlas and the collared flycatcher are significantly darker than the pied flycatchers
and they have larger patches of white on the wing and forehead. Moreover, the two former
species have black tails and a greyish white rump, whereas the pied flycatcher has a white-
edged tail and brownish rump. The Atlas flycatcher has a very large forehead patch that is
even significantly larger than that of the collared flycatcher. The maximum wing patch width
is intermediate between that of the collared and the pied flycatcher and significantly different
from both of them. One interesting feature is that some Atlas flycatchers exhibit a partial neck
collar, similar to that found in semicollared flycatchers and hybrids between the pied and
collared flycatcher.
Table 6 – Plumage characteristics Atlas flycatcher Collared flycatcher Pied flycatcher a Pied flycatcher b Character n=35 n=72 n=41 n=487 Colour score a 1.3 (1.1-1.5) 1.2 (1.1-1.3) 6.3 (6.1-6.6) 3.4 (3.3-3.5) Forehead patch height (mm) 9 (8.6-9.4) 7.7 (7.4-8.0) 1.5 (1.0-2.0) 3.6 (3.5-3.7) Median no. of white-edged tail feathers 0 0 6 (no data) Max. wing patch width (mm) 7.3 (6.7-7.9) 10 (9.5-10.6) 3.8 (3.3-4.2) (no data) Median start of wing patch (primary no.) 4 3 6 (no data) Rump colour Greyish white Greyish white Brown Brown Neck collar No/Partial Yes No No
a Scored according to the seven-point scale of (Drost 1936) where 1=100% black, and 7=100% brown upper parts. Plumage characteristics of Atlas flycatcher males (mean and 95% confidence intervals, unless otherwise stated). Measures from collared flycatchers from Czech Republic and pied flycatchers from Czech Republic (a) and Norway (b) published in (Sætre et al. 2001b).
24
Discussion
The evolutionary history of Atlas flycatcher Both phylogenetic reconstructions and the isolation model fitting suggest that the Atlas
flycatcher originated from a larger ancestral population at approximately the same time as the
collared and the pied flycatcher. This is likely to be in connection with the onset of the
glaciations of the Pleistocene (Sætre et al. 2001a; Sætre et al. 2001b). Since these species of
flycatchers all breed in mixed or deciduous forests (Lundberg and Alatalo 1992), and breed in
nest holes, they are all confined to old forest. They are therefore vulnerable to changes in the
distribution of such forests. During the Pleistocene, the distribution of such forests varied
extensively, but certain areas, so-called refugia, were relatively stable. The main refugia were
Iberia, Italy, the Balkans and Caucasus, and this has been affirmed by several phylogenetic
studies (e.g. DumolinLapegue et al. 1997; King and Ferris 1998). North-western Africa has
also been pointed out as a refugium (Helbig and Seibold 1999). It has been suggested that the
ancestral flycatcher was isolated in four of these refugia during the Pleistocene (Sætre et al.
2001a), and from this isolation diverged into the four present species. This is also consistent
with where they now have their most southern (or only, in the case of the Atlas flycatcher)
distribution, with the semicollared flycatcher in the Balkans or Caucasus, the pied flycatcher
in Italy, the collared flycatcher in Iberia and the Atlas flycatcher in the Atlas Mountains of
north-western Africa.
The D-tests and F-test are slightly negative in two of the species (the Atlas and the collared
flycatcher) although not significantly different from zero. Negative test statistics can be a
result of a bottlenecked expansion, and this might coincide with the same isolation during the
glaciations in the Pleistocene. Negative test statistics is also expected under purifying or
positive selection (e.g. Li 1997). As mentioned above, a selective sweep would show the same
pattern. An earlier study suggested that recurrent selective sweeps have occurred on the Z-
chromosome in collared and pied flycatcher (Borge et al. 2005). They found loss of genetic
variation at Z-linked loci that exceeded what could be ascribed to demographic processes, as
also found significant deviations from neutral expectation at Z-linked loci according to an
HKA-test. In the present study I found reduced variation at the Z-linked loci compared to the
autosomal loci also in the Atlas flycatcher, which is consistent with selective sweeps. On the
other hand, test of neutrality, including HKA-tests, were not significant. Hence my data
25
apparently do not suggest the action of selective sweeps. Note, however, that the HKA-test
was not significant for the pied flycatcher and collared flycatcher in the present study. One
likely explanation for this apparent discrepancy between my study and that of Borge et al.
(2005) is that I analysed fewer gene fragments than they did. Hence, further studies of the
Atlas flycatcher using more genes may be advisory.
The morphology of the Atlas flycatcher Very few studies of the plumage characteristics of the Atlas flycatchers have been done.
Lundberg and Alatalo (1992) measured a few museum specimens caught in North Africa.
However, such museum collections might contain transmigrants in addition to local breeders,
and these measures are therefore uncertain. Sætre et al. (2001b) included statistics of plumage
characteristics of 5 males caught on the breeding grounds in their study, but with the addition
of 30 new males in the present study, it is possible to give a more thorough comparison of the
phenotypic variation of the species. By comparing the Atlas flycatcher with other allopatric
populations it is possible to assess which features are ancestral. It therefore seems like dark
colour and a partial neck colour could be regarded as ancestral traits since they can be found
in collared flycatchers, Atlas flycatchers, semicollared flycatchers and also in pied flycatchers
from Spain (Potti and Merino 1995). The very large forehead patch of the Atlas flycatcher
might be a result of sexual selection as has been found for the forehead patch of the collared
flycatcher (Sheldon and Ellegren 1999).
Traces of introgression? Phylogenetic reconstructions based on sequences from different parts of the genome can be
used to infer previous episodes of introgression (Sætre et al. 2003). According to Haldane’s
rule (Haldane 1922) hybrids of the heterogametic sex are more often sterile or inviable than
those of the homogametic sex. Moreover, genes affecting hybrid fitness and sexually selected
traits are often sex linked (Reinhold 1998; Noor et al. 2001; Sætre et al. 2003). Since females
are the heterogametic sex in birds, they are the ones to suffer the most lowered fitness in the
case of hybridisation. Thus introgression through fertile males may continue to occur at
autosomal loci after it has stopped at maternally inherited mtDNA and at sex-linked loci. In
hybrids between pied and collared flycatchers, females have greatly reduced fitness owing to
almost complete sterility, whereas male hybrids are sometimes fertile (Veen et al. 2001).
26
Accordingly, in hybrid zones between these two species, there is basically no introgression
occurring at Z-linked loci or mtDNA, whereas autosomal introgression occurs at a low rate
(Sætre et al. 2003). At present, the breeding range of the Atlas flycatcher is separated from
those of the pied and collared flycatcher only by small bodies of water at Gibraltar and
between Tunisia and Sicily. Hence interbreeding and introgression could occur or could have
done so in the past. If so, one would expect potential introgression between Atlas flycatcher
and other flycatchers to express the same pattern as found in current hybrid zones of pied and
collared flycatchers, which would have resulted in a greater genetic similarity between the
species at autosomal loci than at Z-linked loci. This was found not to be the case. In all
phylogenetic reconstructions the Atlas flycatcher formed a clade equally distinct as those of
the pied and collared flycatcher, and there was no indication of a relative greater genetic
resemblance at autosomal loci than at Z-linked loci. Finally, higher levels of introgression at
autosomal loci than at Z-linked loci would result in more variation being accumulated at the
former loci. However, the Atlas flycatcher was found to possess a high level of variation at
the Z-linked loci compared to that found in the other two species and compared to the
variation found at autosomal loci. It is therefore no indications in my data set that there has
been any historic or recent introgression between the Atlas flycatcher and any of the two other
species.
My results are consistent with earlier studies using mtDNA (Sætre et al. 2001a; Sætre et al.
2001b). In all trees the Atlas flycatchers appear as one distinct clade with bootstrap values
higher than 96, indicating that the Atlas flycatcher is a monophyletic group. Since no traces of
gene flow could be found and since the genetic distance to its closest relatives is as high as
that between the pied and the collared flycatcher which evidently exhibit strong reproductive
isolation in sympatry, I suggest that the Atlas flycatcher should remain being classified as a
distinct species.
Reduced variation at Z-linked loci The various tests performed showed a reduced level of variation at the Z-chromosome
compared to autosomal loci. This might just reflect that the Z-chromosome has an effective
population size, Ne, which is ¾ of the Ne of autosomal loci since females only have one copy
of the chromosome. One would expect genetic drift to reduce variation and fix shared
polymorphisms between two diverging populations faster in small populations than in large
27
populations (Wakeley and Hey 1997). Another explanation for the lowered level of variation
at Z-linked loci is a smaller male population size. Due to female heterogamy, a female-biased
operational sex ratio would lower Ne at Z-linked loci more than at autosomal. A female-
biased operational sex ratio has been found in the pied and the collared flycatcher due to
greater variance in male than in female reproductive success. Male flycatchers are often
polygynous, attracting a secondary or sometimes even a tertiary female after their first mate
have initiated egg laying. The proportion of secondary females in collared flycatchers has
been found to be around 4% (Qvarnstrom et al. 2003) and the equivalent number in pied is on
average 10-15% (Lundberg and Alatalo 1992). The mean rate of extra pair paternity (EPP),
another measure of the operational sex ratio, was found to be 14,5% in collared flycatcher
(Veen et al. 2001), and slightly lower in pied flycatcher. Unfortunately, similar data are not
available for the Atlas flycatcher. Yet, due to its close resemblance and genetic relatedness to
the other two species it seems likely that also the Atlas flycatcher may exhibit similar features
and thus have a somewhat female-biased sex ratio that could contribute to the observed loss of
genetic variation at Z-linked loci.
Under a neutral model, assuming random mating, constant population size and no migration,
and with the mutation rate at Z-linked and autosomal loci being the same, the ratio of neutral
variation between Z-linked and autosomal loci is predicted to be 0.75. With the most extreme
form of female-biased operational sex ratio, where one male fathers all the chicks, this ratio
would be 0.5. The predicted male-biased mutation rate caused by different numbers of cell
divisions in the germ line, however, would most likely result in this ratio being higher. In this
study I found the ratio of Z-linked to autosomal variation to be 0.60, 0.48 and 0.51 in the
Atlas, collared and pied flycatchers respectively (based on θW). These values are slightly
higher for collared and pied flycatcher than what has previously been found (Borge et al.
2005), but still close to the expected minimum under the most extreme female-biased
operational sex ratio. These low values of variation may indicate that processes such as
selective sweeps has contributed to the loss of variation after all, even though I could not
detect it statistically in the neutrality tests. Further studies with a higher statistical power
(more markers and/or larger sample size) are needed to test this possibility thoroughly.
When two populations become isolated from each other we assume that there will, by random
genetic drift, be a reduction in the amount of shared polymorphism and an increase in the
amount of fixed differences over time. This process occurs faster when Ne is low, and
28
therefore we expect it to occur faster at Z-linked loci since they have a Ne that is ¾ that of
autosomal loci because of the mode of inheritance of the Z chromosome. I found such a
pattern, with a high amount of fixed differences at Z-linked loci and a rather high amount of
shared polymorphisms at autosomal loci (figure 2). The isolation model fitting still indicated
that these values are consistent with the isolation model of speciation, meaning that these
patterns of polymorphism and divergence could be explained without invoking selection
and/or gene flow. Historical changes in θ could generate a greater degree of stochastic
variance in these patterns (Borge et al. 2005). These historical changes in θ might be a result
of changes in Ne.
With a higher mutation rate in males, because of more cell divisions in the male germ line per
generation compared to the female germ line, one would expect a higher divergence at Z-
linked loci compared to autosomal loci (Haldane 1935). Since the Z chromosome spends 67
% of its time in the male germ line (where the mutation rate is assumed to be high) and 33 %
of its time in the female germ line, compared to the autosomal chromosomes that spend 50 %
in each germ line, a correspondingly large different mutation rate should be expected. In
several bird species a higher mutation rate at the Z-chromosome than at autosomes consistent
with a male biased mutation rate has been found (Ellegren and Fridolfsson 1997; Kahn and
Quinn 1999; Carmichael et al. 2000). But even though this difference in the number of germ
line cell divisions may be the predominant cause of the difference in mutation rate across the
genome, other factors might be of importance at a more local scale, such as regional variation
in mutation rate, adaptive variation in mutation rate, or gene specific deviations from
neutrality (Ellegren 2007). In the present study quite closely related species were compared.
In species that diverged recently, much of the observed polymorphisms would be mutations
accumulated before the species diverged. We would expect these levels of polymorphisms to
differ between different genomic regions, e.g. between autosomal loci and Z-linked loci
(Ellegren 2007). This difference in levels of ancestral polymorphisms would then make it
difficult to obtain an accurate estimate of the male-to-female mutation rate ratio, as seen
between humans and closely related apes (Makova and Li 2002).
Borge et al. (2005) found a higher divergence at Z-linked loci than at autosomal loci of
collared and pied flycatcher. This was not confirmed in the present study. The reason for this
discrepancy may partly be because Borge et al. (2005) analysed a larger data set than I have
and thus more accurate estimates. Moreover they restricted the analysis to intronic data only.
29
In my analysis I have included flanking exonic sequence. Exons tend to evolve more slowly
than introns, and might therefore contribute to explain why I did not get the expected result.
Indeed the Z-linked loci I used contained more exon sequences than did the autosomal ones.
Deviations from neutrality As mentioned above, the test statistics from Tajima’s D and Fu and Li’s D and F are for Atlas
and collared flycatcher overall slightly negative, and for pied slightly negative in autosomal
loci. Negative values mean an excess of rare variants, which can result from purifying or
positive selection or a population expansion (Tajima 1989a; Braverman et al. 1995; Simonsen
et al. 1995). We are to expect more frequent selective sweeps at the Z-chromosome, due to
lower recombination rate and because recessive alleles are not masked by dominance in
females (Charlesworth et al. 1987; Begun and Whitley 2000; Sætre et al. 2003). The present
study found no significant deviation from neutrality, but Borge et al. (2005) found traces of
recurrent selective sweeps in their study. At Z-linked loci in the pied flycatcher, however, the
test statistics are mainly positive. Positive test statistics, an excess of intermediate-frequency
variants, could be a result of balancing selection or certain demographic processes such as
recent establishment of secondary contact of differentiated population (Tajima 1989a;
Simonsen et al. 1995). Fu and Li’s tests are strongly dependent on the number of singleton
polymorphisms and the almost complete lack of singleton sites at the Z-linked loci in the pied
flycatcher samples might be causing the significant deviation from neutral expectations. The
positive values might be caused by a recent reduction in population size, that have affected
the Z-linked loci differently from the autosomal, as seen between human mtDNA and nuclear
loci (Fay and Wu 1999).
Conservation issues Since the Atlas flycatcher lives in a fairly limited geographic area, mainly consisting of small
fragmented patches of forest, it may be a species needing special attention for conservation. A
study focusing on conservation issues of the barbary macaques (Macaca sylvanus) from the
same forest these Atlas flycatchers were sampled from, reported a significant increase in
degradation of oak and cedar forest attributed to a significant increase in grazing animals
since 1994 (Ciani et al. 2005). This degradation of oak forest would represent fragmentation
and a loss of habitat, and is a potential threat to the Atlas flycatcher. Further, not much is
30
known about the status of the wintering grounds of the Atlas flycatcher in tropical Africa. The
Atlas flycatcher is not present in the IUCN Red list (IUCN 2006). However, its status as
distinct species is of new date (Sætre et al. 2001b) and it is likely that it is classified as a
subspecies of pied flycatcher in the mentioned report, a species that is assigned to the Least
Concern category. If the Atlas flycatcher is severely affected by habitat loss and
fragmentation one might expect it to show low genetic variation due to a reduced effective
population size. The Atlas flycatcher, however, shows relatively high levels of variation,
almost as high as the collared flycatchers analysed here, which were sampled from a large
population in Italy in apparently non-fragmented habitat (Abruzzo National Park). The pied
flycatcher from Spain has lower variation, and therefore presumably a smaller effective
population size, Ne, than the two other species. Indeed the pied flycatcher population
analysed here if from fragmented mountain forests of Central Spain (Potti and Montalvo
1991). In short, my data on genetic variation of the Atlas flycatcher appears to be consistent
with a genetically healthy population of relatively large size. However, with further
degradation of the forest habitats in the Atlas Mountains this situation could change rapidly.
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